Modular percutaneous spinal fusion

ABSTRACT

A posterior spinal fusion system may include pedicle screws and one or more segments designed to be attached to the implanted pedicle screws via nuts to fuse spinal motion segments in a modular fashion. The pedicle screws may have semispherical receiving surfaces, and each segment may have mounting portions at both ends, with corresponding semispherical engagement surfaces. The nuts may also have semispherical surfaces. The semispherical surfaces permit polyaxial adjustment of the relative orientations of the segments and pedicle screws. Each mounting portion may have a passageway therethrough to receive the proximal end of the corresponding pedicle screw; each passageway may intersect the edge of the corresponding engagement surface to facilitate percutaneous placement. Such placement may be carried out through cannulae to provide a minimally invasive (MIS) implantation procedure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of:

pending U.S. patent application Ser. No. 11/464,161, filed Aug. 11, 2006which carries Applicants' docket no. MLI-43 NPROV and is entitledMODULAR PERCUTANEOUS SPINAL FUSION SYSTEM AND METHOD, which claims thebenefit of:

U.S. Provisional Application No. 60/708,006, filed Aug. 11, 2005, whichcarries Applicants' docket no. MLI-43 PROV, and is entitled PEDICLESCREW AND POLYAXIAL SECUREMENT SYSTEM FOR SPINAL POSTERIORSTABILIZATION.

The foregoing are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to implantable devices, and moreprecisely, to posterior spinal fusion systems.

2. The Relevant Technology

Many people experience joint pain in one form or another. In particular,back pain may result from the occurrence of a wide variety of spinalpathologies. Some such pathologies are currently treated by fusingadjacent vertebrae to prevent their relative motion. According to oneknown method, pedicle screws are implanted in the pedicles and arerigidly secured to a rod passing posterior to the pedicles.

Unfortunately, current procedures often involve the exposure of arelatively large area to permit implantation of the rod. Such exposurecauses patient discomfort and lengthens the post-surgical healing time.

Some current procedures cannot be used to implant a rod that securesmore than two vertebrae together. Other procedures can be used to fusemultiple vertebral levels, but such procedures often require difficultand potentially inaccurate rod contouring procedures to maintain theproper spinal curvature. Other known procedures are somewhat complex,and therefore require many parts and surgical steps. Accordingly, thereis a need for new fusion systems and methods that remedy theshortcomings of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is an exploded, perspective view of a spinal fusion systemaccording to one embodiment of the invention.

FIG. 2 is a perspective view of the spinal fusion system of FIG. 1 infully-assembled form, secured to three vertebrae of a spine.

FIG. 3 is a side elevation, section view of the spinal fusion system andthe vertebrae of FIG. 2.

FIG. 4 is a perspective view of a segment of a spinal fusion systemaccording to one alternative embodiment of the invention.

FIG. 5 is an exploded, perspective view of a spinal fusion systemaccording to another alternative embodiment of the invention.

FIG. 6 is a side elevation, section view of a patient's lumbar region,with cannulae and the pedicle screws of the spinal fusion system of FIG.5.

FIG. 7 is a side elevation, section view of the lumbar region andcannulae, with the pedicle screws and the first segment of the spinalfusion system of FIG. 5.

FIG. 8 is a side elevation, section view of the lumbar region andcannulae, with the pedicle screws and the inferior segment of the spinalfusion system of FIG. 5, and an additional segment similar to theinferior segment.

FIG. 9 is a side elevation, section view of the lumber region andcannulae, with the entire spinal fusion system of FIG. 5 in place on thevertebrae of the lumbar region, except that the superior segment of FIG.5 has been replaced by the additional segment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to spinal fusion systems and methods thatare modular and/or percutaneously implantable. The drawings andaccompanying description are merely exemplary. Accordingly, the scope ofthe present invention is not intended to be limited by the examplesdiscussed herein, but only by the appended claims.

Referring to FIG. 1, an exploded, perspective view illustrates a spinalfusion system 10 according to one embodiment of the invention. Thespinal fusion system 10, or system 10, may be used for patients withchronic spinal problems including damaged intervertebral discs, spinalstenosis, facet degeneration, and spondylolisthesis. Arrows in FIG. 1indicate the orientation of the system 10 upon implantation on thespine, with reference to conventional anatomical directions. Moreprecisely, the arrows illustrate a cephalad direction 12, a caudaldirection 14, an anterior direction 16, a posterior direction 18, and amedial/lateral axis 20.

As shown, the system 10 includes a first fixation member 24, a secondfixation member 26, a third fixation member 28, a superior securementsection 30, an inferior securement section 32, a first nut 34, a secondnut 36, and a third nut 38. Since, in the embodiment of FIG. 1, thesystem 10 is designed for posterior fusion, i.e., fusion of vertebraevia attachment to their posterior aspects, each of the fixation members24, 26, 28 may take the form of a pedicle screw. Accordingly, thefixation members 24, 26, 28 will hereinafter be termed first, second,and third pedicle screws 24, 26, and 28. Furthermore, since the system10 is designed to provide modular fusion, i.e., fusion along a variablenumber of spinal motion segments, the first and second securementsections 32, 34 may be termed first and second segments 32, 34.

Each of the pedicle screws 24, 26, 28 may have a proximal end 42, adistal end 44, and a receiving flange 46 positioned between the proximalend 42 and the distal end 44. Each proximal end 42 may have a torquereceiver 48 and a threaded portion 50. The torque receivers 48 aredesigned to be engaged by a distal end of a tool (not shown) so thattorque can be transmitted from the tool to the pedicle screws 24, 26, 28to drive the pedicle screws 24, 26, 28 into bone. The threaded portions50 are designed to receive the nuts 34, 36, 38 so that the nuts 34, 36,38 can secure the segments 30, 32 to the pedicle screws 24, 26 28 in amanner that will be shown and described subsequently.

The distal ends 44 are threaded so that the distal ends 44 can bethreadably implanted in bone. The pedicle screws 24, 26, 28 mayoptionally be cannulated to facilitate guidance of the pedicle screws24, 26, 28 into engagement with the vertebral pedicles via guide wiresor other implements (not shown). The receiving flanges 46 protruderadially from the main bodies of the pedicle screws 24, 26, 28 toprovide receiving surfaces 52 capable of attachment to the segments 30,32 at multiple relative orientations. The receiving surfaces 52 may besemispherical, and in the embodiment of FIG. 1, the receiving surfaces52 are also concave.

The cephalad segment 30 may have a cephalad mounting portion 56, acaudal mounting portion 58, and a stem 60. The cephalad mounting portion56 is attachable to the second pedicle screw 26 and the caudal mountingportion 58 is attachable to the first pedicle screw 24. The stem 60connects the cephalad mounting portion 56 to the caudal mounting portion58. The stem 60 may optionally be deformable to permit adjustment of therelative positions and/or orientations of the mounting portions 56, 58.However, since the mounting portions 56, 58 are polyaxially adjustablyattachable to the fixation members 24, 26, deformation of the stem 60may not be needed.

As shown, the cephalad mounting portion 56 has a concave engagementsurface 62 and a convex engagement surface 64. Each of the engagementsurfaces 62, 64 has a semispherical shape capable of polyaxiallyadjustable engagement with a corresponding semispherical shape. Moreprecisely, the concave engagement surface 62 may engage a convex surfacewith a similar radius of curvature at any of a plurality of relativeorientations about three orthogonal axes. Similarly, the convexengagement surface 64 may engage a concave surface with a similar radiusof curvature at any of a plurality of relative orientations about threeorthogonal axes. If desired, the engagement surfaces 62, 64 may havesubstantially the same radius of curvature so that the cephalad mountingportion 56 is able to polyaxially adjustably engage a mounting portionsimilar to itself from either side.

The cephalad mounting portion 56 also has a passageway 66 capable ofreceiving the proximal end 42 of the second pedicle screw 26. Thepassageway 66 takes the form of a slot with a rounded interior end. Thepassageway 66 intersects the lateral edge of the cephalad mountingportion 56 so that the proximal end 42 can be inserted into thepassageway 66 not only via relative anterior/posterior motion betweenthe proximal end 42 and the passageway 66, but alternatively, viarelative medial/lateral motion between the proximal end 42 and thepassageway 66.

The caudal mounting portion 58 has a configuration somewhat similar tothat of the cephalad mounting portion 56. More precisely, the caudalmounting portion 58 has a concave engagement surface 72 and a convexengagement surface 74. The engagement surfaces 72, 74 are semisphericaland may optionally have the same radius of curvature. The caudalmounting portion 58 also has a passageway 76 capable of receiving theproximal end 42 of the first pedicle screw 24. However, unlike thepassageway 66 of the cephalad mounting portion 56, the passageway 76does not intersect the lateral edge of the caudal mounting portion 58.Rather, the passageway 76 is fully bounded. Thus, the proximal end 42must be inserted into the passageway 76 via relative anterior/posteriormotion between the proximal end 42 and the passageway 76.

The caudal segment 32 may be configured in a manner somewhat similar tothat of the cephalad segment 30, and may be designed to operate in asimilar manner. As shown, the caudal segment 32 also has a cephaladmounting portion 86, a caudal mounting portion 88, and a stem 90 thatconnects the mounting portions 86, 88 together.

The cephalad mounting portion 86 has a concave engagement surface 92, aconvex engagement surface 94, and a passageway 96. Like the engagementsurfaces 62, 64, 72, 74, the engagement surfaces 92, 94 may besubstantially semispherical, and may have the same radius of curvature.The cephalad mounting portion 86 has a passageway 96 therethrough, whichis capable of receiving the proximal end 42 of the first pedicle screw24. Unlike the passageways 66, 76, the passageway 96 takes the form of aslot that intersects the cephalad edge of the cephalad mounting portion86. Accordingly, the proximal end 42 can be inserted into the passageway96 not only via relative anterior/posterior motion between the proximalend 42 and the passageway 96, but alternatively, via relativecephalad/caudal motion between the proximal end 42 and the passageway96.

The caudal mounting portion 88 may be substantially identical to thecaudal mounting portion 86. The caudal mounting portion 88 may thus havea concave engagement surface 102, a convex engagement surface 104, and apassageway 106, which are all substantially the same as theircounterparts 92, 94, 96 of the cephalad mounting portion 86. Thus, theproximal end 42 of the third pedicle screw 28 can be inserted into thepassageway 106 not only via relative anterior/posterior motion betweenthe proximal end 42 and the passageway 106, but alternatively, viarelative cephalad/caudal motion between the proximal end 42 and thepassageway 106.

The nuts 34, 36, 38 are designed to engage the proximal ends 42 to pressthe associated mounting portions 56, 58, 86, 88 against the receivingflanges 46 of the corresponding pedicle screws 24, 26, 28. Morespecifically, each of the nuts 34, 36, 38 has a threaded bore 110, apolygonal perimeter 112, and a convex compression surface 114. Thethreaded bores 110 are designed to engage the threaded portions 50 ofthe proximal ends 42. The polygonal perimeters 112 are shaped to beengaged by a distal end of a tool (not shown) so that the tool can beused to rotate the threaded bores 110 into engagement with the threadedportions 50. The polygonal perimeters 112 may be hexagonal. The convexcompression surfaces 114 may be substantially semispherical in shape,and may have a radius of curvature similar to those of the concaveengagement surfaces 62, 72, 92, 102 of the segments 30, 32.

The system 10 may be assembled in multiple different configurations. Asshown in FIG. 1, in one exemplary mode of assembly, the inferior segment32 is first positioned such that the convex engagement surfaces 94, 104engage the receiving surfaces 52 of the receiving flanges 46 of thefirst and third pedicle screws 24, 28. Then, the superior segment 30 ispositioned such that the convex engagement surface 74 engages theconcave engagement surface 92 of the cephalad mounting portion 86 of theinferior segment 32, and the convex engagement surface 64 engages thereceiving surface 52 of the receiving flange 46 of the second pediclescrew 26.

Notably, the semispherical shapes of the engagement surfaces 62, 64, 72,74, 92, 94, 102, 104 and receiving surfaces 52 enables the orientationsof the segments 30, 32 to be polyaxially adjusted relative to each otherand to the pedicle screws 24, 26, 28. This polyaxial adjustability ispossible about any of three orthogonal axes for each engaging pair ofsemispherical surfaces. Thus, the configuration of the system 10 can beadjusted to suit a wide variety of spinal morphologies without requiringdeformation of any of the components 24, 26, 28, 30, 32, 34, 36, 38 ofthe system. In this application, the phrase “polyaxially adjustability”refers to the ability of one member to be rotated relative to a secondmember in at least two orthogonal axes, and then attached to the secondmember in the selected relative orientation.

After the pedicle screws 24, 26, 28 and segments 30, 32 have beenpositioned and adjusted as described above, the nuts 34, 36, 38 arerotated into engagement with the threaded portions 50 and tightened tosecure the segments, 30, 32 to the pedicle screws 24, 26, 28. Moreprecisely, the nuts 34, 36, 38 press the convex engagement surfaces 94,64, 104 against the receiving surfaces 52 of the first, second, andthird pedicle screws 24, 26, 28, respectively, and press the convexengagement surface 74 against the concave engagement surface 92. Thus,further adjustment of the orientations of the segments 30, 32 isprevented, and the system 10 is made substantially rigid to preventrelative motion between the associated vertebrae.

Referring to FIG. 2, a perspective view illustrates the spinal fusionsystem 10 of FIG. 1 in fully-assembled form, secured to a portion of aspine 120. The spine 120 has a first vertebra 122, a second vertebra 124superior to the first vertebra 122, and a third vertebra 126 inferior tothe first vertebra 122. The first vertebra 122 has a right pedicle 132,and second vertebra 124 has a right pedicle 134, and the third vertebra126 has a right pedicle 136.

In order to secure the system 10 to the portion of the spine 120, thepedicle screws 24, 26, 28 may first be implanted in the pedicles 132,134, 136, respectively, through the use of methods known in the art. Ifdesired, guide wires (not shown) may first be implanted in the pedicles132, 134, 136. The pedicles 132, 134, 136 may be reamed or otherwiseresected to remove interfering bone. Then, the cannulated pedicle screws24, 26, 28 may be inserted over the guide wires to facilitateimplantation of the pedicle screws 24, 26, 28 into the pedicles 132,134, 136. Once the pedicle screws 24, 26, 28 have been placed, thesegments 30, 32 are placed and adjusted as described previously. Then,the nuts 34, 36, 38 are tightened on the proximal ends 42 to secure thesegments 30, 32, as also described above.

FIG. 2 illustrates usage of the system 10 to provide only unilateralfixation. However, those of skill in the art will recognize that asimilar system (not shown) may be also attached to the left pedicles(not visible) of the vertebrae 122, 124, 126 to provide bilateralfixation.

Referring to FIG. 3, a side elevation, section view illustrates thesystem 10 secured to the vertebrae 122, 124, 126 of FIG. 2. FIG. 3 showsthe distal ends 44 of the pedicle screws 24, 26, 28, which are implantedin the pedicles 132, 134, 136 of the vertebrae 122, 124, 126. FIG. 3also illustrates the manner in which the engagement surfaces 62, 64, 72,74, 92, 94, 102, 104, the receiving surfaces 52, and the convexcompression surfaces 114 engage each other. As in FIG. 2, the nuts 34,36, 38 have been tightened so that the system 10 is substantially rigid,and the vertebrae 122, 124, 126 are unable to move relative to eachother.

Referring to FIG. 4, a perspective view illustrates a segment 150 of aspinal fusion system (not shown) according to one alternative embodimentof the invention. As shown, the segment 150 has a shape similar to thatof the superior segment 30 of the system 10 of FIG. 1. Accordingly, thesegment 150 has a cephalad mounting portion 156, a caudal mountingportion 158, and a stem 160 that connects the mounting portions 156, 158together.

The cephalad mounting portion 156 has a concave engagement surface 162and a convex engagement surface 164, which may be substantiallysemispherical, and may have the same radius of curvature. The cephaladmounting portion 156 has a passageway 166 like the correspondingpassageway 66 of the superior segment 30 of FIG. 1. Accordingly, thepassageway 166 intersects a lateral edge of the cephalad mountingportion 156.

However, the concave engagement surface 162 differs from that of thesuperior segment 30 of FIG. 1 in that a plurality of surfaceirregularities 168 are present on the concave engagement surface 162.The surface irregularities 168 may take the form of bumps as shown, ormay instead be divots, ridges, grooves, peaks, or any other featurescapable of enhancing frictional force between the concave engagementsurface 162 and the surface it engages. If desired, features like thesurface irregularities 168, or differently shaped features, may bepresent on the convex engagement surface 164 to enhance frictionalengagement of the convex engagement surface 164.

The caudal mounting portion 158 has a concave engagement surface 172 anda convex engagement surface 174, which may be substantiallysemispherical, and may have the same radius of curvature. The caudalmounting portion 158 has a passageway 176 like the correspondingpassageway 76 of the superior segment 30 of FIG. 1. Accordingly, thepassageway 176 is fully bounded, and does not intersect any edge of thecaudal mounting portion 158.

However, the concave engagement surface 172 differs from that of thesuperior segment 30 of FIG. 1 in that a plurality of surfaceirregularities 178 are present on the concave engagement surface 172Like the surface irregularities 168, the surface irregularities 178 maytake the form of bumps as shown, or may instead be divots, ridges,grooves, peaks, or any other features capable of enhancing frictionalforce between the concave engagement surface 172 and the surface itengages. If desired, features like the surface irregularities 178, ordifferently shaped features, may be present on the convex engagementsurface 174 to enhance frictional engagement of the convex engagementsurface 174.

If desired, the surface irregularities 168 and/or the surfaceirregularities 178 may provide a pattern of radial or otherwiseevenly-spaced ridges, grooves, and/or other features that provide aclocking feature. Such a clocking feature may limit engagement of thecorresponding engagement surfaces 162, 164, 172, 174 with their opposingsurfaces to a plurality of discrete relative positions and/ororientations.

In any case, the surface irregularities 168, 178 may help to providemore secure engagement between the segment 150 and any other pediclescrews, nuts, and/or segments to which it is secured. Thus, the segment150 may help resist slippage of the corresponding system after thesystem has been locked into the desired configuration.

Referring to FIG. 5, an exploded, perspective view illustrates a spinalfusion system 210 according to another alternative embodiment of theinvention. Like the system 10 of FIG. 1, the system 210 includes a firstpedicle screw 224, a second pedicle screw 226, a third pedicle screw228, a superior segment 30, an inferior segment 32, a first nut 234, asecond nut 236, and a third nut 238. The segments 30, 32 may beidentical to those of FIG. 1. Overall, the system 210 is similar to thesystem 10, except that in the system 210, the segments 30, 32 areoriented such that the concave engagement surfaces 62, 64, 72, 74, 92,94, 102, 104 are oriented toward the pedicle screws 224, 226, 228.

As shown, each of the pedicle screws 224, 226, 228, has a proximal end242, a distal end 244, and a receiving flange 246 between the proximaland distal ends 242, 244 Like the proximal ends 42 of the pedicle screws24, 26, 28 of FIG. 1, each of the proximal ends 242 has a torquereceiver 48 and a threaded portion 50. As in FIG. 1, the torquereceivers 48 receive torque from a tool to facilitate implantation ofthe pedicle screws 224, 226, 228, and the threaded portions 50threadably receive the nuts 234, 236, 238.

Each of the receiving flanges 246 has a receiving surface 252 with asubstantially semispherical shape. However, unlike the receivingsurfaces 52, the receiving surfaces 252 are convex. Therefore, thereceiving surfaces 52 are shaped to engage the concave engagementsurfaces 62, 72, 92, 102 of the segments 30, 32.

Like the nuts 34, 36, 38 of FIG. 1, each of the nuts 234, 236, 238 has athreaded bore 310 and a polygonal perimeter 312. However, in place ofthe convex compression surfaces 114, each of the nuts 234, 236, 238 hasa concave compression surface 314. The concave compression surfaces 314are shaped to engage the convex engagement surfaces 64, 74, 94, 104 ofthe segments 30, 32.

Like the system 10, the system 210 may be assembled in multipledifferent configurations. As shown in FIG. 5, the superior segment 30may first be positioned such that the concave engagement surfaces 62, 72engage the receiving surfaces 252 of the receiving flanges 246 of thefirst and second pedicle screws 224, 226. Then, the inferior segment 32is positioned such that the concave engagement surface 92 engages theconvex engagement surface 74 of the caudal mounting portion 58 of thesuperior segment 30, and the concave engagement surface 102 engages thereceiving surface 252 of the receiving flange 246 of the third pediclescrew 228. As in FIG. 1, the semispherical shapes of the engagementsurfaces 62, 64, 72, 74, 92, 94, 102, 104 and receiving surfaces 252enables the orientations of the segments 30, 32 to be polyaxiallyadjusted relative to each other and to the pedicle screws 224, 226, 228.

After the pedicle screws 224, 226, 228 and segments 30, 32 have beenpositioned and adjusted as described above, the nuts 234, 236, 238 arerotated into engagement with the threaded portions 50 and tightened tosecure the segments, 30, 32 to the pedicle screws 224, 226, 228. Thenuts 234, 236, 238 then press the concave engagement surfaces 72, 62,102 against the receiving surfaces 252 of the first, second, and thirdpedicle screws 224, 226, 228, respectively, and press the concaveengagement surface 92 against the convex engagement surface 74. Thus,the system 210 is made substantially rigid to prevent relative motionbetween the associated vertebrae.

Either of the systems 10, 210 may be implanted in an open accesssurgical procedure. However, it may be desirable to useminimally-invasive surgical (MIS) techniques to reduce the disruption tosurrounding tissues. Accordingly, it may be advantageous to implant oneor more components of the systems 10, 210 percutaneously. “Percutaneousimplantation” refers to motion of an implant to an implantation sitewithin the body, in which at least some of the tissues covering theimplantation site remain intact. FIGS. 6 through 9 illustrate oneexemplary procedure for implanting a system similar to the system 210percutaneously.

Referring to FIG. 6, a side elevation, section view illustrates apatient's lumbar region. As shown, the patient has skin 320, in which afirst incision 322, a second incision 324, and a third incision 326 havebeen formed. The incisions 322, 324, 326 are appropriately sized toreceive a first cannula 328, a second cannula 330, and a third cannula332, respectively. Each of the cannulae 328, 330, 332 has a bore 334extending along its length, a cephalad slot 336, and a caudal slot 338.The slots 336, 338 extend proximally from the distal ends of thecannulae 328, 330, 332. In the alternative to the cannulae 328, 330,332, any type of cannula known in the art, and more particularly usedfor MIS surgery, maybe used.

As an initial step in the implantation method, guide wires (not shown)may be inserted through the skin at the sites where the incisions 322,324, 326 are to be formed, and implanted into the pedicles 132, 134, 136of the vertebrae 122, 124, 126, respectively. Dilators or other devices(not shown) may be used to widen the guide wire entry points to form theincisions 322, 324, 326. The distal ends of the cannulae 328, 330, 332may then be inserted over the protruding proximal ends of the guidewires and through the incisions 322, 324, 326 to maintain access to thepedicles 132, 134, 136.

Once the cannulae 328, 330, 332 have been placed, the pedicle screws224, 226, 228 of the system 210 of FIG. 5 may be implanted in thepedicles 132, 134, 136. More precisely, the first pedicle screw 224 maybe inserted over the corresponding guide wire and through the firstcannula 328 until the distal end 244 of the first pedicle screw 224rests on the pedicle 132 of the first vertebra 122. The first pediclescrew 224 may then be rotated and urged toward the pedicle 132 throughthe use of a tool (not shown) until the first pedicle screw 224 has beenimplanted to the proper depth. The second and third pedicle screws 226,228 may be implanted in the pedicles 134, 136 of the second and thirdvertebrae 124, 126 through the second and third cannulae 330, 332,respectively, through the use of similar procedures.

Referring to FIG. 7, a side elevation, section view illustrates thelumbar region, cannulae 328, 330, 332, and pedicle screws 224, 226, 228.After the pedicle screws 224, 226, 228 have been implanted in thepedicles 132, 134, 136, the inferior segment 32 may be inserted throughthe third cannula 332, with the cephalad mounting portion 86 leading. Asthe cephalad mounting portion 86 reaches the slots 336, 338 of the thirdcannula 332, the inferior segment 32 may be rotated such that thecephalad mounting portion 86 passes through the cephalad slot 336 of thethird cannula 332, and into the first cannula 328 through the caudalslot 338 of the first cannula 328. This rotation may be accomplishedthrough the use of a variety of implements (not shown), such as rampformed in or inserted into the third cannula 332 to direct the cephaladmounting portion 86 toward the first cannula 328, or a rigid orarticulating gripping tool that enables the surgeon to control theorientation of the inferior segment 32.

Since the passageway 96 of the cephalad mounting portion 86 intersectsthe cephalad edge of the cephalad mounting portion 86, the proximal end42 of the first pedicle screw 224 is able to slide into the passageway96 as the passageway 96 moves along the cephalad direction relative tothe proximal end 42. The edge of the cephalad mounting portion 86 thenslides along the receiving surface 252 of the first pedicle screw 224.The caudal mounting portion 88 may then be dropped onto the thirdpedicle screw 228 such that the proximal end 42 of the third pediclescrew 228 enters the passageway 106 of the caudal mounting portion 88.

Referring to FIG. 8, a side elevation, section view illustrates thelumbar region and cannulae 328, 330, 332, with the pedicle screws 224,226, 228 and the inferior segment 32 in place. After the inferiorsegment 32 has been positioned as described above, an additional segmentsubstantially identical to the inferior segment 32 may be insertedthrough the first cannula 328, with the cephalad mounting portion 86leading. The segment 32 may then be positioned on the first and secondpedicle screws 224, 226 in substantially the same manner as the inferiorsegment 32 that has already been implanted to rest on the first andthird pedicle screws 224, 228.

More precisely, as the cephalad mounting portion 86 reaches the slots336, 338 of the first cannula 328, the inferior segment 32 may berotated such that the cephalad mounting portion 86 passes through thecephalad slot 336 of the first cannula 328, and into the second cannula330 through the caudal slot 338 of the second cannula 330. The proximalend 42 of the second pedicle screw 226 is able to slide into thepassageway 96 of the cephalad mounting portion 86 as the passageway 96moves along the cephalad direction relative to the proximal end 42. Theedge of the cephalad mounting portion 86 then slides along the receivingsurface 252 of the second pedicle screw 226. The caudal mounting portion88 may then be dropped onto the cephalad mounting portion 86 of thepreviously implanted inferior segment 32 such that the proximal end 42of the first pedicle screw 224 enters the passageway 106 of the caudalmounting portion 88.

Referring to FIG. 9, a side elevation, section view illustrates thelumber region and cannulae 328, 330, 332, with the entire spinal fusionsystem in place. After the inferior segment 32 and the additionalsegment 32 have been positioned relative to the pedicle screws 224, 226,228 and adjusted to their proper orientations relative to each other andthe pedicle screws 224, 226, 228, the nuts 234, 236, 238 may be appliedto secure the inferior segment 32 and the additional segment 32 to thepedicle screws 224, 226, 228.

More specifically, the first nut 234 may be inserted through the firstcannula 328 such that the threaded bore 310 of the first nut 234receives the threaded portion 50 of the proximal end 42 of the firstpedicle screw 224. The first nut 234 is then tightened. In similarfashion, the second and third nuts 236, 238 are inserted through thesecond and third cannulae 330, 332 and tightened on the threadedportions 50 of the proximal ends 42 of the second and third pediclescrews 226, 228. Thus, the fixation system is substantially rigid, andfurther adjustment between any of the receiving surfaces 252 and theengagement surfaces 92, 94, 102, 104 is substantially prevented.

The cannulae 328, 330, 332 are then removed from the incisions 322, 324,326, respectively, and the incisions 322, 324, 326 are closed to permitthe implantation site to commence healing. Those of skill in the artwill recognize that FIGS. 6 through 9 illustrate only one of manypotential MIS procedures. Such procedures maybe used with a wide varietyof embodiments, including the systems 10, 210 of FIGS. 1 and 5 and thesegment 150 of FIG. 4.

The foregoing description discloses a number of different elements thatmay be combined in various ways to provide a number of alternativeimplantable systems. Although the foregoing examples relate toimplantation of a posterior spinal fusion system, the present inventionmay be applied to a wide variety of implants, within and outside theorthopedic area.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. It isappreciated that various features of the systems and methods describedabove can be mixed and matched to form a variety of other alternatives.As such the described embodiments are to be considered in all respectsonly as illustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A system comprising: a first fixation member implantable in a firstvertebra; a superior segment comprising a caudal mounting portionattachable to the first fixation member, the caudal mounting portioncomprising an engagement surface; and an inferior segment comprising acephalad mounting portion attachable to the first fixation member, thecephalad mounting portion comprising an engagement surface; wherein theengagement surfaces of the caudal and cephalad mounting portions areshaped to polyaxially adjustably engage each other while the engagementsurfaces are in engagement with one another.
 2. The system of claim 1,further comprising: a second fixation member implantable in a secondvertebra superior to the first vertebra; and a third fixation memberimplantable in a third vertebra inferior to the first vertebra; whereinthe superior segment comprises a cephalad mounting portion attachable tothe second fixation member, and the inferior segment comprises a caudalmounting portion attachable to the third fixation member such that, uponattachment to the first, second, and third fixation members, the firstand second segments substantially prevent relative motion between thefirst, second, and third vertebrae.
 3. The system of claim 2, whereinthe caudal mounting portions of the superior and inferior segments havesubstantially the same shape and the cephalad mounting portions of thesuperior and inferior segments have substantially the same shape.
 4. Thesystem of claim 2, further comprising an additional segment comprising acaudal mounting portion attachable to the second fixation member,wherein the engagement surface of the cephalad mounting portion of thesuperior segment is shaped to polyaxially adjustably engage anengagement surface of the caudal mounting portion of the additionalsegment.
 5. The system of claim 1, wherein each of the engagementsurfaces comprises a generally semispherical shape with a passagewaytherethrough, wherein each of the passageways is sized to receive aproximal end of the first fixation member.
 6. The system of claim 5,further comprising a nut shaped to engage the proximal end to attach themounting portions to the first fixation member.
 7. The system of claim6, wherein the first fixation member comprises a receiving surfaceshaped to receive either of the cephalad mounting portion and the caudalmounting portion such that tightening of the nut presses the mountingportion against the receiving surface.
 8. The system of claim 1, whereineach of the superior segment and the inferior segment is percutaneouslyimplantable through a cannula; and wherein at least one of the superiorsegment and the inferior segment is adjustable through a cannula topolyaxially adjust the engagement surfaces.
 9. The system of claim 8,wherein each of the engagement surfaces comprises a slot extendingtherethrough to permit entry of a proximal end of the first fixationmember into the slot in response to motion of the slot along a directiongenerally perpendicular to an axis of the first fixation member.
 10. Thesystem of claim 1, wherein at least one of the engagement surfacescomprises at least one surface irregularity shaped to restrict relativemotion between the engagement surfaces in response to pressure urgingthe engagement surfaces together.
 11. The system of claim 1, wherein thefirst fixation member comprises a pedicle screw implantable in a pedicleof the first vertebra, wherein the superior and inferior segments areshaped to bridge intervertebral spaces between the pedicle of the firstvertebra and pedicles of two vertebrae adjacent to the first vertebra.12. A system comprising: a first fixation member implantable in a firstvertebra; a second fixation member implantable in a second vertebrasuperior to the first vertebra; a third fixation member implantable in athird vertebra inferior to the first vertebra; a superior segmentcomprising a caudal mounting portion rigidly attachable to the firstfixation member and a cephalad mounting portion attachable to the secondfixation member; and an inferior segment comprising a cephalad mountingportion rigidly attachable to the first fixation member and a caudalmounting portion attachable to the third fixation member; wherein atleast one of the superior and inferior segments is percutaneouslyimplantable.
 13. The system of claim 12, wherein the caudal mountingportions of the superior and inferior segments have substantially thesame shape and the cephalad mounting portions of the superior andinferior segments have substantially the same shape.
 14. The system ofclaim 12, wherein each of the mounting portions comprises an engagementsurface comprising a generally semispherical shape with a passagewaytherethrough, wherein each of the passageways is sized to receive aproximal end of the corresponding fixation member.
 15. The system ofclaim 14, further comprising first, second, and third nuts shaped toengage the proximal ends to attach the mounting portions to the first,second, and third fixation members, wherein each of the fixation memberscomprises a receiving surface shaped to receive at least one of themounting portions such that tightening of the nut presses the mountingportion against the receiving surface.
 16. The system of claim 15,wherein each of the receiving surfaces comprises a generallysemispherical shape.
 17. The system of claim 12, wherein at least one ofthe mounting portions comprises a slot extending therethrough to permitentry of a proximal end of the corresponding fixation member into theslot in response to motion of the slot along a direction generallyperpendicular to an axis of the corresponding fixation member.
 18. Thesystem of claim 12, wherein at least one of the mounting portionscomprises an engagement surface comprising at least one surfaceirregularity shaped to restrict relative motion between the engagementsurface and an engagement surface of an adjacent mounting portion. 19.The system of claim 12, wherein the first, second, and third fixationmembers comprise pedicle screws implantable in pedicle of the first,second, and third vertebrae, respectively, wherein the superior andinferior segments are shaped to bridge intervertebral spaces between thepedicles of the first, second, and third vertebrae.
 20. The system ofclaim 12, wherein the caudal mounting portion of the superior segmentcomprises an engagement surface and the cephalad mounting portion of theinferior segment comprises an engagement surface, wherein the engagementsurfaces of the caudal and cephalad mounting portions are shaped topolyaxially adjustably engage each other.